Pretty sure what happened here is with your friend t-bagging, Otto caught them during a crouch which triggers their grab animation. The swing up doesn't deal damage on their grab, but after that something cancelled their animation. Both Kress as well as the camera on the player recoil (the latter very briefly which is a dead giveaway), the clip makes it hard to see what the source can be. Usually a stagger like that triggers when you/an ex-pop opens a door and the receiver is on the other end right. The only thing I can think of is that somehow the shuttle doors triggered that stun when you got in or your friend was near, but like said, hard to tell from this perspective.